This invention relates to spinner cones used in aircraft applications and is more particularly related to the method of manufacturing a spinner cone having a metal outer shell with an underlying thermoplastic advanced composite.
Spinner cones are located at the front of aircraft engines. The design criteria for a spinner cone are weight, dynamic strength, and impact resistance. Impact resistance is critical due to birds or other foreign objects striking the spinner cones. Obviously due to the aircraft application, weight is of great concern and the lighter the spinner cones, the more desirable its design.
Most spinner cones produced today are manufactured from two materials. The first materials are metallic materials traditionally used in aircraft applications. The cones are formed in traditional metal forming ways such as spinning if working with relatively thin metal plates or forging and machining. This latter process is preferable when thicker gauge spinner cones are required, but this process is also more expensive and time consuming.
A second group of materials are thermoset composites which offer weight and cost savings as compared to metal spinner cones. Generally speaking, thermoset matrix composite spinner cones do not offer high impact resistance without significantly increasing their thickness, which in turn increases their weight. Some thermoset composites will give the desired weight and impact resistance, but at the expense of other desirable characteristics. For example, glass/epoxy or Kevlar aramid can be used to produce a desired weight and impact resistance of a finished spinner cone. Generally speaking, Kevlar/epoxy has a high impact strength, but is much less capable, especially in compression loads than glass/epoxy. Carbon/epoxy is very infrequently used in spinner cone applications because one of the primary criteria for a spinner cone is impact resistance. Carbon/epoxy is typically very strong, but also very brittle and is not capable of withstanding impact loads like glass/epoxy or Kevlar/epoxy.
To fabricate a spinner cone from glass/epoxy or Kevlar/epoxy, a semi-cylindrical or gore section which has been cut from the preimpregnated sheet stock material is placed in a female mold portion which has been prepared by applying a mold release to the inner surface of the mold. The mold is most frequently made of aluminum. The semi-cylindrical or gore sections are placed in the mold and the seams are staggered with successive plies placed in the mold until the desired laminate thickness is achieved. Once the hand lay-up is completed, the mold and part are placed in a vacuum bag and a vacuum is applied to it. The mold and part are then placed into an autoclave where pre-determined pressure and temperature are applied for a suitable time necessary to polymerize the resin system and consolidate the part into a structural component. The temperature normally used for these thermoset materials is either 250.degree. F. or 350.degree. F., depending upon the epoxy system. An aluminum tool is suitable for use in this temperature range.
Thermoplastic advanced composites have significant property advantages over thermoset systems such as an impact resistance of up to 10 times higher, improved microcrack resistance, negligible moisture absorption, superior flame and radiation resistance, and excellent damping characteristics. They require no refrigeration and do not change properties after extended storage. Parts made from thermoplastic composites may be reheated and reformed, scrap can be recycled, and no toxic emissions are produced during processing.
However, structural thermoplastic materials require approximately 700.degree. F. for a processing consolidation temperature. This temperature requirement limits the number of mold materials suitable for use in production. Aluminum is no longer a viable option. Instead, materials such as electroless nickel-plated molds or steel molds must be used. The fabrication of high temperature molds is usually expensive and require a long lead-time for development.
Cast ceramic molds processing temperatures, and can be fabricated in short lead times. A disadvantage of cast ceramic molds are that they are likely to delaminate or degrade at the surface. This is caused during the molding of parts and results from the resins ingressing into micropores or microcracks in the surface of the ceramic mold. When the part is stripped from the mold, portions of the ceramic mold are stripped with the part causing degradation of the mold's surface. After a relatively few number of parts are manufactured using this process, the surface of the ceramic mold is effectively destroyed.
Applicant's inventive method provides a means of allowing inexpensive and quickly produced cast ceramic molds to be used in processing advanced thermoplastics matrix composites.
In Applicant's inventive process, a thin metal layer or shell is applied to the surface of the ceramic mold. The thickness of the metal shell and the metal selected is dependent upon the characteristics desired of the final finished spinner cone. The thermoplastic composite is then applied over the metal shell to its required thickness. The mold is then closed and heated. The pressure and temperature cause the polymers in the resin system to consolidate into the part, bond with the metal layer and form the final structural component. The hybrid metal/thermoplastic matrix composite part has numerous advantages over the present state-of-the-art thermoset or metal spinner cones.